We have discovered a preparation of spontaneously beating cells from adult mammalian myocardium which are devoid of any measureable membrane potential. We have shown that the sarcolemmae of these cells is structurally intact, although permeable to calcium, ATP, ADP and lanthanum. These cells appear to be ideal for studies of the intracellular metabolism of calcium as well as other metabolic processes. We have recently shown that calcium influx into these cells is rapid enough to provide all the calcium used for contraction. This calcium remains sequestered in intracellular organelles, and the efflux rate is very slow compared to influx. This argues in favor of extracellular sources for the origin of contractile calcium, and makes a regenerative release mechanism unlikely. In other studies we have used these cells to evaluate in situ mitochondrial function. We have found that oxidative phosphorylation is intact in this preparation, and that calcium acts to inhibit oxygen uptake. The future experiments will determine if there is a threshhold concentration of calcium in the medium which allows contraction, or if contractile force is proportional to concentration over a wide range. This will provide further insight into inotropic mechanisms.